Jerry Tyson, also known as 3XK, or Triple Killer, got his nickname by strangling 3 women in a week, dropping out of sight for a month, and then killing three more. Then he disappeared. 4 years later, the Triple Killer is back. (first appearance on the show : episode 6, season 3).
Beckett and Castle investigate the new 3XK murders. The victims : Linda Russo, and Kim Foster. No one knows the true identity of 3XK when episode 3x06 starts. Beckett finds evidence that leads her to believe 3XK is a man named Marcus Gates, an ex-convict, that just got out one month before Linda Russo's murder, after having served 4 years in prison. During the course of the investigation, Beckett and Castle get to meet Jerry Tyson in prison, and interrogate him about Gates, not knowing Tyson is the real 3XK. Tyson gives them more evidence to make them believe they got the real killer. When they think that the case is over, Castle finds out that it was a setup, and that when Gates was still in prison, Tyson had told him everything he needed to know to take the fall for the Triple Killer murders. Tyson then admits to Castle that Gates was the perfect choice because he likes prison. Tyson could then have been cleared of all suspicion. Having almost completely served his time in prison, and having obtained an early release for his cooperation on the case, Jerry decides to take things into his own hands when he realizes that Castle found out the truth. He ties him to a chair and runs away, just before the police storms into the room where Castle is held. Tyson's plan with this setup was to stay under the radar, and to make the police believe the killer had been caught, so that when he finally gets out of prison, nothing could tie him to the 3XK murders. Thanks to Castle, his plan fell through. Tyson is now on the run. It has been confirmed that the 3XK storyline will be continued in season 4 (episode 4).
-------- [Tyson's victims : -> 4 years earlier : Lauren Brackett, Sheri Ort, Melanie Sherman, Rachel Gold, Sara Townsend, Emma Keener. -> 4 years later : Linda Russo, Kim Foster.]
-------- [Tyson's modus operandi : After strangling them, he poses the bodies of his victims as if they were lying in a coffin. All his victims are blondes.]
-------- [Tyson's backstory : Raised by a single mother. She was blonde (like Tyson's victims). She never wanted him. She died from drug overdose when Jerry was 12. He went into foster care. According to Castle, Jerry kills those women to get back at his mother, because of all the hate he has towards her for abandoning him. But he leaves them looking peaceful, because as much as he hated his mother, he loved his mother.]